Vertical transmission of HIV is a major problem in many parts of the world, but particularly in Africa. Estimates of transmission rates have ranged from 13% to 48% in various studies, but little is known about the relative contributions of in utero, perinatal, and postnatal transmission. Recent studies have demonstrated the occurrence of postnatal transmission, presumably through breast milk, in the setting of acute maternal postpartum infection. The objective of the current proposal is to determine the frequency of breast milk transmission of HIV among infected mothers to their infants in Nairobi, Kenya. The study will be conducted as a prospective randomized clinical trial comparing infant HIV infection rates between breast-fed and formula-fed groups. After informed consent is obtained, pregnant HIV seropositive women attending an antenatal clinic will be randomly assigned to one of two groups. Group I women will be counselled to breast-feed their infants in accordance with current Kenyan Ministry of Health and World Health Organization recommendations. Group II women will be counselled to formula feed and a supply of formula will be provided. Women will be evaluated for demographic and medical historical variables, HIV related disease stage, CD4 levels, and quantitative HIV titers in peripheral blood lymphocytes (PBLs) and plasma using polymerase chain reaction (PCR) amplification. Infants will be evaluated from birth to two years for morbidity, mortality, and presence of serum HIV IgG and IgA, and HIV DNA in PBLs using PCR assays. The presence and quantity of HIV DNA and RNA in colostrum and breast milk will be assessed among breast feeding mothers. The rate of HIV infection in infants in Group I and Group II will be compared and an attributable risk defined. The association between HIV infection in infants and potential correlates, including maternal viral titers in PBLs and breast milk, CD4 levels, HIV related disease, duration of exclusive vs partial breast feeding, nipple pathology, or infant oral pathology will be evaluated. Excess morbidity and mortality attributable to formula feeding will be defined after stratifying infants into infected and uninfected groups. Knowledge regarding the frequency of breast milk transmission of HIV will be valuable for counselling pregnant women, formulation of public health policy in Kenya and other countries, and development of rational intervention strategies.